Intermittent Air Conditioning Problem
 

Hi All,

I have a 99 Sierra 4WD with the Z71 package. Recently the A/C started cutting out intermittently at idle. While cruising around town the A/C works great, but at idle the compressor is shutting off.

I would like to get a scan tool that will give me the enhanced data, but do not want to spend an arm and a leg for a Snap On scanner. Can anyone recommend a reasonably priced scan tool that will give the enhanced data?

Thanks,
Ken

The a/c compressor will cycle on and off as needed...If you are getting cold air, its fine

A scan tool isn't gonna tell you why your compressor is turning off and on. Your HVAC isn't part of your 1999 Chev's operating network.

All A/C systems cycle like that at idle. Your A/C is operating normally

The system is not working normally. I have a thermometer in the air vent, and I can watch the temperature go from a cool 45 degrees while driving around up to 80 or 90 degrees when I get to a stop light. But this is only sometimes because the problem is intermittent.
The truck in the new body style for 1999. I have the factory manuals where the first step of every diagnostic is to run a complete scan using a scan tool. The diagnostics for the A/C system tell me to run a scan, and the electrical diagram shows me an AC module and all the sensors going into the VCM. So I have every reason to believe that the A/C system is part of the computer network.
I know that the clutch is not engaged when I have this problem, and I know that the low pressure switch is not the problem. Beyond that I have not been able to consistently probe around while the problem is occurring. Hence the need for a scan tool.
Thanks,
Ken

this won't be much data you can access on a 99 for a/c even with a factory scan tool. newer models will show you the reason a/c request is withheld but I don't think a 99 will. a set of pressure gauges would give you more diagnostic information.

I would start by recovering the system to see if the charge is low. even 0.4 of a lb loss of refrigerant will cause the system to short cycle. If the charge is good view the high and low side pressures. I'm wondering if the condenser fins are full of debris restriction air through the condenser. If the system can't transfer the heat out, the high pressure cut off will shut down the compressor.

I was having this problem with my 2000 sierra. After verifying the charge level, monitoring the pressures and washing debris from my condenser the problem was still there. It seemed the more the system was used, the less intermittent the problem was.


I decided to change the low pressure cycling switch. At the parts counter, the parts guy noted that there was an attached tsb for this part. bulletin 03-01-39-007 addresses intermittent a/c operation, with the low pressure cycling switch being a possible fault. Even though the tsb is for model years 2002 to 2003, I bought and installed the updated switch which was listed for my 2000.


The updated gm part # is 89040362; my cost was $37. Its a simple screw in installation as the accumulator fitting has a Schrader valve; so the system does not need to be evacuated.


I will post back if it fixes it.

after a 2 hour drive the a/c was ice cold the entire trip. fixed

some pics